Explanation

 Given:

6 men × 8 hours/day = total man-hours per day = 48 man-hours

They earn Rs. 720 per week

Now:

8 men × 6 hours/day = 48 man-hours per day — same total man-hours as before

So, since the total man-hours per day (and thus per week) are the same, the weekly earnings will also be the same.